The benefits of using solar PV energy to power your immersion
Posted by Nick Mills on 26 November 2012 at 10:27 am
We all hate the though of giving something away for nothing, and, although we get paid to generated solar electricity, and get an additional nominal sum for exporting the surplus, do you ever dream of getting a little bit more from your solar PV system?
There are now a few systems on the market which you can connect to your immersion heater and use to heat your water when there is excess solar-generated energy available which would otherwise be exported to the grid. These units monitor the amount of electricity generated as well as the amount that your home is using. When there is a surplus this energy is diverted into your immersion cylinder instead of being exported to the grid. You still get paid for the energy you generate as well as for the half that is deemed to be exported. (1)
While it is very hard to quantify the exact payback of these systems, there are some assumptions that we can make:
• In summer, when most PV is available, the primary heater for the water (gas/oil/wood) is likely to be off, and the main energy source will be electricity.
• The average house with a 4kWp PV system will export roughly 50% of this, on an annual basis, more in the summer, less in the winter.
• A 10 minute shower will consume aprox. 1.5kWh of electricity. (2)
The standard hot water cylinder is 150 litres, and the energy required to heat the water in it from, say 30 to 80 degrees, (or 10 to 60 degrees) will be 8.75kWh. (3)
A 4kWp solar PV system will produce up to around 25kWh on a good day, so should be more than capable of providing all the hot water required. Of course, not all days will produce this surplus, and on some days, the tank will already be hot enough without the need to top it up with ‘free’ solar PV energy.
So, with all the variables such as the number of occupants, the numbers of showers they take, and for how long, how sunny it is, the size of the tank, and of the PV array, and of course the amount of energy they would otherwise export, it is almost impossible to make a financial assessment of the viability of installing this type of system.
However, if we work on average solar PV generation, we can look at the summer months and see that from April to October (when the primary heating is likely to be off), PV generation on a 4kWp system will average around 16kWh a day, or about 7-8kWh exported. If all this heat is used, (some of it, about 150W, wont be used directly – it will ‘leak’ into the airing cupboard where it may be diminished, but still useful), then the value of the electricity can be valued at, say 12p/kWhr. This equates to about £150/year. There will also be some additional savings in the winter, as well as potentially more in the summer if less than 50% of the PV energy is used on site first.
As the installed cost of these units is around £500 for a professionally fitted system, this represents excellent value for money, and should be considered by anyone who has a reasonably sized PV system, who doesn’t already use all their PV energy, and who has a use for hot water!
(1) Assumes that no export tariff has been arranged with your supplier. This is true in the majority of cases but some customers have an agreement to sell the export to their supplier, rather than being paid for ‘deemed’ export.
(2) Assumes 9kW electric shower comparison.
(3) Dr. Nick Jenkey, MIET, Dulas LtdPhoto: Immersun solar PV load diversion unit
More information about using PV generated electricity on site from YouGen
About the author: Nick Mills is solar project manager at Dulas Ltd
If you have a question about anything in the above blog, please ask it in the comments section below.
20 comments - read them below or add one